Drona is great Brahmin acharya (professor) for Pandavas, Kauravas and many other Kshatriyas of that time. He has revenge on Drupada due to insult made by Drupada and hence Dronacharya took revenge on Drupadha.
Chanakya is a famous Brahmin mentor for Chandragupta. He has revenge on Dhana nanda due to insult made by Dhana nanda and hence chanakya took revenge on Dhana nanda.
There are many such famous 'take revenge's' in mythology and in Indian history.
I have doubt regarding this take revenge stories. Is taking revenge for some insult a sin or not?
What if some one excuses other in-spite of getting insult? Is it dharma to take revenge for getting insult?